In a method which is described in CH-B-647 999, winding signals are produced from yarn pulses and are then counted. In practice, the correct number of winding signals sometimes represents a number of withdrawn windings that is either too great or too small, resulting in picks that are too short or too long. For it sometimes happens that a free lint cluster or a yarn component hanging onto the yarn (e.g. in multifilament yarns) is dragged behind a withdrawn yarn winding and below the withdrawal sensor and therebeyond, so that the sensor reports the cluster or component as an additional winding. By contrast, when two adjacent windings are withdrawn in close vicinity below the winding sensor, only one winding signal is produced for the two windings.
EP0 286 584 B1 discloses another method of this kind, in which yarn pulses of a plurality of circumferentially distributed withdrawal sensors are converted into winding signals, then supplied to an evaluation unit and compared with an expected signal pattern which corresponds to a predetermined time sequence of the winding signals during interference-free operation. The winding signals are only taken into account for the control of the weft-yarn store when the time sequence of the supplied winding signals complies with the expected pattern.
Furthermore, prior use in practice has revealed a method in which each yarn pulse is converted into a winding signal in a filter means assigned to the receiver of the withdrawal sensor and in which the winding signal helps to open a time window within which successive pulses or signals are ignored. Such a measure prevents lint clusters following at a lower speed from leading to winding signals within the time window. However, when two closely adjacent windings are withdrawn, then the second winding can no longer be detected, which leads to an excessively long pick.
In modern, fast air-jet weaving machines it sometimes happens for reasons which are not exactly understood that, for instance, once every 1000 insertion cycles there is an insertion in the case of which the yarn is inserted at a slower pace than has been predetermined. Such an insertion, however, is not to effect a shut-off of the weaving machine, for the insertion is per se correct, but only too slow. Furthermore, it has been found in practice that with specific yarn qualities not only lint clusters are separately entrained after the yarn windings, but yarn components which are still clinging to the yarn are also dragged along, e.g., in the case of multifilament yarns. Such lint clusters or clinging components then produce other types of pulses (with flat ramp and low frequency content) than the yarn itself. Such false yarn pulses caused by entrained yarn components are also not meant to produce winding signals. By contrast, two winding signals are actually to be produced when two windings are simultaneously withdrawn, as is sometimes the case. These above-mentioned circumstances create special requirements for the withdrawal sensor which is to make a reliable distinction between yarn windings and other objects.
It is the object of the present invention to provide a simple method of the above-mentioned kind and a yarn withdrawal sensor for performing said method with the help of which short and long picks are avoided when using a measuring and feeding device a weaving machine.